Attention has lately been focused on an OCDM method as a multiplexing method suited for attaining higher speed and higher capacity of an optical metro-access network. The OCDM method is a method of implementing multiplexing by encoding/decoding respective channels at a transmitter and a receiver, respectively, with the use of code groups orthogonal to each other.
As described hereunder, as the method of implementing encoding/decoding, there is available a method of time spread/wavelength hopping, using the so-called chirped Fiber Bragg Grating (hereinafter referred to as FBG), advantageous in terms of ease in implementation and manufacturing cost. The chirped Fiber Bragg Grating is made up of a plurality of different diffraction gratings, formed in the longitudinal direction of a fiber.
First, a process of encoding/decoding by time spread/wavelength hopping, as disclosed in JP, 2000-209186, A, is described with reference to FIG. 2. In FIG. 2, there is shown a case where a data period Tb is equal to a code period Tc by way of example, however, as shown in “Enhancement of Transmission Data Rates Incoherent FO-CDMA Systems” by X. Wang and K. T. Chang, OECC 2000, 14A2-5, p. 458 (2000), even if the data period Tb differs from the code period Tc, encoding/decoding is enabled.
On a transmitting side, sending data 101 in the form of optical signals are inputted to an encoder 103 as shown in FIG. 2(a1). The sending data 101 (102) made up of optical signals with a predetermined number N1 (3 in FIG. 2) of wavelengths at λ1 to λ3 are subjected to intensity modulation according to the return-to-zero (RZ) format, in accordance with sending data made up of electric signals, creating valid data in a time slot (chip) for each of the data periods Tb as shown in FIG. 2(a2). Respective wavelength components included in the sending data 101 are delayed (encoded) by specified time in accordance with a specified coding pattern (Code 1) at the encoder 103, respectively, to be thereby turned into an optical signal 105 with waveforms spread on a time axis as shown in FIG. 2(a3).
The optical signal 105 obtained after undergoing time spread by delay time corresponding to the respective wavelength components arrives at a decoder 106 via a transmission line 104. At the decoder 106, the respective wavelength components of the optical signal 105 as inputted are delayed (decoded) by the specified time in accordance with the specified coding pattern (Code 1), and as shown in FIG. 2(a4), there is obtained received data 107 (108) identical to the original sending data 101, with the respective wavelength components multiplexed within the same chip period after undergoing de-spread (delay time differences for the respective wavelength components are cancelled out) on the time axis.
FIGS. 2(b1) through 2(b4) show a case where the specified coding pattern of the encoder 103 differs from that of the decoder 106. Accordingly, if the coding patterns, for the transmitter and receiver, respectively, are found identical to each other by comparing the received data 107 (108), made up of optical signals, after subjected to, for example, photoelectric conversion, with a threshold value, original information group (sending data made up of electric signals) can be taken out while if the coding patterns, for the transmitter and receiver, respectively, differ from each other, the original information group cannot be taken out.
Further, even in the case of multiplexing the optical signal 105 of respective channels, subjected to time spread/wavelength hopping, at a multiplexer, and sending the same out to a transmission line, even if a multiplexed optical signal is given to the decoder 106 of a channel via a demultiplexer, it is possible to take out only desired received data (desired information group) matching a receiver's own coding pattern provided that orthogonality of the coding patterns is maintained. As is clear from description of the above-described principles of transmission, in the case of the time spread/wavelength hopping method, the optical signal 105 having a plurality of wavelengths needs to be transmitted.
However, because an optical fiber serving as a transmission line has chromatic dispersion characteristics, the optical signal arrives at the decoder 106 with various propagation time differences occurring among the respective wavelength components. Accordingly, there can occur a case where decoding cannot be properly implemented. Furthermore, in the case of a transmit/receive system executing multiplex transmission over a plurality of channels, the orthogonality between coding patters collapses due to the chromatic dispersion characteristics of an optical fiber, raising the risk of adversely affecting other channels.
In order to obviate such inconvenience, there is the need for compensating for propagation time difference for respective wavelength components, occurring due to the chromatic dispersion characteristics, by separate means. As a method of compensating for chromatic dispersion, a method whereby a dispersion compensation fiber and a phase conjugating device are inserted in a transmission line, and others have already been applied to many optical transmission systems.
In any case, however, problems have arisen in that there is an increase in the number of components, and in the scale of the OCDM encoder (transmitter) and/or OCDM decoder (receiver), resulting in higher cost of apparatuses.
Accordingly, it is highly desired to provide an optical transmitter, optical receiver, and optical transmission system, having a configuration capable of canceling out chromatic dispersions of a transmission line but capable of minimizing the scale of system elements, and reducing cost of manufacturing.